Truong Thi Nha stands just four-and-a-half feet tall. Her three grown children tower over her, just as many young people in this village outside Hanoi dwarf their parents.

The biggest reason the children are so robust: fertilizer.

Nha, her face weathered beyond its 51 years, said her growth was stunted by a childhood of hunger and malnutrition. Just a few decades ago, crop yields here were far lower and diets much worse.

Then the widespread use of inexpensive chemical fertilizer, coupled with market reforms, helped power an agricultural explosion that had already occurred in other parts of the world. Yields of rice and corn rose, and diets grew richer. Now those gains are threatened in many countries by spot shortages and soaring prices for fertilizer, the most essential ingredient of modern agriculture.

Prices of some fertilizer have nearly tripled in the last year, keeping farmers from buying all they need. That is one of many factors which has contributed to the rise in food prices that, according to the UN World Food Programme, threatens to push tens of millions of poor into malnutrition.

Protests over high food prices have erupted across the developing world, and the stability of governments from Senegal to the Philippines is threatened.

In the US, farmers in Iowa, desperate to replenish nutrients in the soil, have increased the age-old practice of spreading tonnes of hog manure on their fields. In India, the cost of subsidizing fertilizer for farmers has soared, sparking calls for policy reform.

The squeeze on the supply of fertilizer has been building for roughly five years. Rising demand for food and biofuels prompted farmers everywhere to plant more crops. As demand grew, the fertilizer mines and factories proved unable to keep up.

Fertilizer companies are confident the shortage will be solved eventually, noting that they plan to build scores of new factories in the next few years, many in West Asia where natural gas is abundant. But that will probably create fresh problems in the long run as the world grows more dependent on fossil fuels to produce chemical fertilizers. Intensified use of chemical fertilizers is certain to mean greater pollution of waterways, too.

Agriculture and development experts say the world has few alternatives to its growing dependence?on fertilizer. Some experts calculate that synthetic fertilizers made with natural gas have led to greater crop yields. In sub-Saharan Africa, where hunger and starvation have long been a threat, a lack of fertilizer is a primary reason yields lag behind the rest of the world. Efforts to get fertilizer into the hands of African farmers have been complicated by the recent price increases.

“It’s a very basic and direct arithmetic point that putting fertilizer on the ground on a 1-acre plot can, in typical cases, raise an extra tonne of output,” said Jeffrey D. Sachs, the Col-umbia University economist who has focused on eradicating poverty. “That’s the difference between life and death.” The demand for fertilizer has been driven by a confluence of events, including population growth, shrinking world grain stocks and the appetite for corn and palm oil used to make biofuel. But experts say the biggest factor has been the growing demand for food, especially meat, in the developing world.

Recently, Nha, the tiny Vietnamese woman, stood in a field outside her village, her weather-beaten face shielded from the drizzle by a big straw hat. She took a break from wielding her wood-handled hoe and described the meager diets of her youth.

Her family, including six brothers and sisters, struggled to survive on rations from the commune where they lived, eating little protein. The occasional pigs they raised on rice stalks and mush “fattened very slowly”, Nha recalled.

But market reforms in Vietnam during the last two decades gave farmers access to fertilizer and higher-yielding seeds. Rice yields for each acre have doubled and corn yields have tripled.

Several times a season, Nha and her neighbours walk down their rows of corn with battered metal buckets full of chemical fertilizer,?which looks like coarse grey sand. They sprinkle a bit at the base of each plant and carefully hoe it in. Nha’s husband, Le Van Son, remembers villagers’ amazement in the 1990s when they learnt that a pound (0.45kg) of chemical fertilizer contained more of the major nutrients than 100 pounds of manure.

Overall global consumption of fertilizer increased by an estimated 31% from 1996 to 2008, driven by a 56% increase in developing countries, according to the International Fertilizer Industry Association.

“Markets are asking farmers to step on the accelerator,” said Michael R. Rahm, vice-president for market analysis and strategic planning at Mosaic, a major fertilizer producer based in Plymouth, Minnesota. “They’ve pressed on it, but the market has told them to step on it harder.”

Fertilizer is basically a combination of nutrients added to soil to help plants grow. The three most important are nitrogen, phosphorus and potassium. The latter two have been available for centuries, and today originate from mines. But nitrogen in a form that plants could absorb was scarce, and the lack of nitrogen led to low crop yields for centuries.

That limitation ended in the early 20th century with the invention of a procedure, now primarily fuelled by natural gas, that draws chemically inert nitrogen from the air and converts it into a usable form.

As the use of chemical nitrogen fertilizer spread, it was accompanied by improved plant varieties and greater mechanization. From 1900 to 2000, worldwide food production jumped by 600%. Scientists said that increase was the fundamental reason world population was able to rise to around 6.7 billion today, from 1.7 billion in 1900.

Vaclav Smil, a professor at the University of Manitoba, calculates that without nitrogen fertilizer, there would be insufficient food for 40% of the world’s population, at least based on today’s diets. Other experts have come up with slightly lower numbers.

Initially, much of the increased production of fertilizer went into grains such as wheat and rice that served as the foundation of a basic diet. But recently, with world economic growth at a brisk 5% a year, hundreds of millions of people began earning enough money to buy more meat from animals fattened with grains. That occurred at the same time that rising production of biofuels, such as corn-based ethanol, put new pressure on grain supplies. These factors translated into rising demand and higher prices for fertilizer.

Manufacturers are scrambling to increase the supply. But these projects are expensive and time-consuming, and supplies are expected to remain tight for years.

Fertilizer is vitally important in Iowa, the US state, whose farmers grow more corn than any other state, and depend on fertilizer to greatly increase their yields. But the combination of high prices and spot shortages has forced some farmers to revert to older methods of fertilization, making hog manure a hot commodity. Farmers are cutting deals to have hog barns built on the edges of their corn and soya bean fields to get ready access to manure.

On a tour of his rolling farm in Oxford Junction in eastern Iowa, Jayson Willimack pointed to the future sites of two buildings that will hold 2,400 hogs. Their manure will eventually replace commercial fertilizer on 400 acres, around 10% of his farm, and save him perhaps $50,000 (around Rs20 lakh) annually. “Every little bit helps,” he said.

Such strategy has severe limits—manure contains so little nitrogen that tonnes are required on each acre. That means farmers have little choice but to pay the higher prices for commercial fertilizer. In many countries, those cost increases have so far been offset by record high prices for crops. But fertilizer inflation has created a crisis in countries that subsidize fertilizer use for farmers.

In India, for instance, the government’s subsidy bill could be as high as $22 billion in the coming year, compared with around $4 billion three years ago, and has prompted calls to reform the programme that India depends on to maintain its food supply.

Once new supplies become available, the rising use of fertilizer will still pose difficulties. Environmental groups fear increased use, particularly of nitrogen fertilizer made using fossil fuels. Because plants do not absorb all the nitrogen; much of it leaches into streams and groundwater. That run-off has long been recognized as a major pollution problem, and it is growing as food production increases.

A barometer of that pollution is the rising number of dead zones where rivers meet the sea. In the Gulf of Mexico, for instance, nitrogen run-off from fields in the Corn Belt washes downstream and feeds plant life in the gulf. The algae blooms suck oxygen from the water, killing other marine life.

More than 400 dead zones have been identified, from the coasts of China to the Chesapeake Bay, and the primary reason is agricultural run-off, said Robert J. Diaz, a professor at the Virginia Institute of Marine?Science.?“Nitrogen?is?nitrogen,” Diaz said. “If it’s on land, it produces corn. If it gets in the water, it produces algae.”

Earlier this month, a UN panel called for urgent changes in agricultural practices to make them less damaging. The panel recommended techniques that offer some of the same benefits as chemical fertilizer, such as increased crop rotation using soya beans and other legumes that naturally add some nitrogen to the soil.

But others say those approaches, while helpful, will be insufficient to meet the world’s rapidly rising demand for food and biofuel.

“This is a basic problem, to feed 6.6 billion people,” said Norman Borlaug, an American scientist who was awarded a Nobel Peace Prize in 1970for his role in spreading intensive agricultural practices to poor countries.